The characteristics of mass spectrometry have raised it to an outstanding position among the various analysis methods. It has excellent sensitivity and detection limits and may be used in a wide variety of applications, e.g. atomic physics, reaction physics, reaction kinetics, geochronology, biomedicine, ion-molecule reactions, and determination of thermodynamic parameters (ΔG°f, Ka, etc.). Mass spectrometry technology has thus begun to progress very rapidly as its uses have become more widely recognized. This has led to the development of entirely new instruments and applications.
Development trends have gone in the direction of increasingly complex mass analyzer designs requiring highly specialized components and tight manufacturing tolerances. Longer analysis times are often associated with this increased complexity. This, in turn, requires system designers to make significant design trade-offs between the accuracy of the mass measurements and the time required to obtain those measurements. However, such trade-offs have become increasingly intolerable in the competitive field of drug discovery and analysis. There, mass analyzers must be both highly accurate and provide for a high throughput of analytes.
Several mass analyzer embodiments based on ion separation in the presence of an electric field are illustrated in the figures of U.S. Pat. No. 5,726,448 to Smith et al, the structures of which are hereby incorporated by reference. FIGS. 3–5 of the '488 patent show a first embodiment of a mass analyzer having a mass filter chamber through which only ions of a selected range of mass-to-charge ratios are permitted to pass. In this embodiment, the mass filter chamber includes first and second electrode pairs that are connected to an RF signal source to generate an electric field therebetween. Each pair of electrodes is formed by an opposed pair of conductive plates. The planar faces of the first electrode pair face each other while the planar faces of the second electrode pair likewise face one another. However, the planar faces of the first electrode pair are disposed substantially perpendicular to the planar faces of the second electrode pair. Both the first and second electrode pairs are aligned along the same length of the chamber.
In a further embodiment, shown in FIG. 10 of the '488 patent, the second electrode pair is displaced from the first electrode pair along the length of the mass filter chamber. In all other respects, this embodiment is substantially similar to the one shown in FIGS. 3–5.
In each of the foregoing embodiments, the electric field generated at the second electrode pair is out of phase by π/2 from the electric field generated at the first electrode pair so that the ions are acted upon by at least two distinct, orthogonal electric fields. As predominantly noted in FIG. 3 of the '488 patent, the orthogonal electric fields are preferably sinusoidal in nature and combine to form a rotating electric field.
In operation, each ion enters the mass selection chamber at angles, θ and Φ, with respect to a plane forming the inlet of the chamber. Whether or not the ion passes completely through the mass selection chamber depends on the mass-to-charge ratio of the ion as well as the frequency of the rotating electric field, the amplitude of the rotating electric field, the phase of the electric field at the time that the ion enters the chamber and the entry angles, θ and Φ.
The present inventor has recognized that the existing mass analysis apparatus shown in the '448 patent may be improved in a variety of manners. For example, trade-offs must frequently be made between system throughput and mass resolution/sensitivity when employing existing mass analyzer constructions. Therefore, there is a need for mass analyzer constructions having increased throughput without corresponding sacrifices in manufacturing, mass resolution, and/or mass sensitivity goals. Further, the electrode configuration shown in the '488 patent generates less than optimal electric field shapes that are particularly undesirable when a device of that type is miniaturized.